The Norwegian Trench as a carbon conduit from the North Sea to the deep Atlantic Ocean: insights from ocean glider observations in spring 2024

The North Sea helps mitigate the impact of human activities on Earth’s climate by absorbing carbon dioxide (CO₂) out of the atmosphere. However, the fate of the absorbed CO₂ is poorly constrained: 0-40 % is estimated to be stored in seafloor sediments as organic matter, with the remaining 60-100 % transported by ocean currents out into the Atlantic Ocean. For the latter portion, the depth at which North Sea waters are exported controls the timescale on which the CO₂ is stored, with deeper export constituting a more enduring carbon sink. During a NoSE (North Sea-Atlantic Exchange) project expedition in spring 2024, a season when biological productivity drives high CO₂ uptake, we deployed two ocean gliders in the Norwegian trench, the main conduit for water exchange between the North Sea and Atlantic Ocean, for several weeks. These autonomous underwater vehicles carried sensors to characterise the water mass structure and some key biogeochemical properties for carbon export (e.g., dissolved oxygen). Here, we use these sensor data together with a broader set of water column observations collected during a concurrent research cruise (including nutrients and marine carbonate system parameters) to investigate potential carbon export mechanisms. The trench was vertically stratified with a deep layer containing extra CO₂ that was being transported towards the Atlantic Ocean. However, the stratification was due to relatively warm, salty North Sea waters being trapped beneath a surface layer of colder, fresher waters from the Norwegian fjords. As this surface layer is spatially confined near the coast, rather than extending widely across the Atlantic, the deeper layer might be exposed to the surface soon after exiting the Trench, and thus its extra CO₂ returned to the atmosphere where it can affect Earth’s climate.